The Fischer-Tropsch process can be used for the conversion of hydrocarbonaceous feed stocks into liquid and/or solid hydrocarbons. The feed stock (e.g. natural gas, associated gas and/or coal-bed methane, biomass, residual oil fractions and coal) is converted in a first step into a mixture of hydrogen and carbon monoxide (this mixture is often referred to as synthesis gas or syngas). The synthesis gas is then fed into a reactor where it is converted over a suitable catalyst at elevated temperature and pressure into paraffinic compounds ranging from methane to high molecular weight modules comprising up to 200 carbon atoms, or, under particular circumstances, even more.
Catalysts typically comprise an active portion, such as a metal or metal component, supported on a carrier material, which can be a porous refractory oxide such as titania, silica or alumina. Catalysts deactivate over time, and are therefore replaced periodically in order to maintain a reasonable product yield.
For example, a supported cobalt catalyst is currently used as a catalyst for the Fischer-Tropsch reaction as well as for certain other applications. The catalyst can be poisoned by a number of different species including, for example, sulfur, sodium, nitrogen or carbon containing compounds; all of which de-activate the catalyst. Furthermore the dispersion of the metal or metal component may decrease.
Also, sintering and agglomeration of the support particles reduces the surface area of the support and consequently the activity of the catalyst.
When the economics dictate that the cost of shutting down the reactor and replacing the catalyst is less than the lost revenue due to deactivation of the catalyst, the reactor is shut down and the catalyst replaced. The de-activated catalyst can be treated by, for example, nitric acid to leach out some or, preferably, all of the relatively expensive cobalt which can be recovered and re-used. However, the support is conventionally just disposed of in land-fill sites.